Stand up to what? The Iranian regime and its terrorist proxies are our main adversaries and enemies in the region. There has been no "standing up" and there will be no standing up to that. In fact we will continue to support the destruction of our enemies.
You are living in some parallel universe. Most of Iran has been severely crippled economically and militarily with most of their leadership killed. They have suffered from 100's of billions of USD in economic damage. Close to 10.000 targets have been destroyed in Iran. In recent weeks and days most of their industrial capacity is being destroyed for all to see.
A few caves is not going to change anything let alone drones or ballistic missiles. They are not going to change anything or win even a single battle. The war, by all purposes, is already lost for Iran if the goal was to protect its economy, assets, industrial and military base and people.
KSA/GCC is largely completely unharmed in comparison. In particular KSA.
You are still forgetting the main key thing here. Iran is not winning anything and is never going to defeat the US or Israel. Not only that the US and Israel will never stop before the Iranian regime has either been removed, severely weakened to the degree that it will no longer pose a threat, reformed by force and most importantly until the nuclear topic (supposedly the main reason for this conflict) is solved once and for all. The US and Israel have all the cards and have all the means of upping the ante in this war. Such as targeting civilians directly (as Iran is doing - mostly complete utter failure), massive land invasion (eventually) or the use of tactical nukes.
Iran in return cannot do anything other than what they have already done. The worst thing they can do is damage regional oil and gas infrastructure (which will be rebuilt) and in return all of their oil and gas infrastructure will be destroyed - rendering the main income source of the Iranian regime gone. Which would speed up the Iranian regime collapse. And unlike the GCC (which can not only easily afford to rebuild) Iran cannot and will not be allowed to if the regime somehow remains intact.
And that is without all the other cards that the US and Israel can play such as support of active separatist groups within Iran, activating more sleeper cells (we have already seen videos from inside Iran where armed people are killing iRGC soldiers in broad daylight).
You are beyond deluded if you think that Israel, after all their recent success in the region (militarily) will not pursue their goals until the very end in Iran. They are not a rational actor. As I wrote I have no doubt that they would happily use tactical nuclear weapons on Iran if they ever felt like it or were ever genuinely threatened. Would anyone make them accountable? I highly doubt it.
And for all we know Israel and the US main goal might not even be regime change. Israel as I wrote likely prefers a perpetually failed and weak Iran over another potential regional competitor. Because a regime change in Iran would most likely mean a more pro-Western regime - which in return means massive US/West/Arab/Jewish/Israeli investments into Iran (untapped market). So this might not even be about regime change but castrating Iran and removing the nuclear threat. Or Balkanization or whatever.
The end result will be the same regardless - a destroyed and severely weakened Iran, their proxies included.